Whereas the optical concentration of directional light onto photovoltaic and/or thermal receivers has long been used in industrial-scale solar power installations, small-scale solar installations have consisted almost entirely of plate-type photovoltaic panels without optical concentration. Such panels are expensive because of the large quantities of photovoltaic material they use, and typically require several years of operation just to recover the energy of production of those materials. A solar panel that used optical concentration with photovoltaics (CPV) could potentially provide a much less expensive alternative to plate-type solar panels by greatly reducing the quantities of photovoltaic materials used. Furthermore, a CPV panel might provide a significantly higher efficiency than a comparable plate-type panel by allowing the economic use of special high-efficiency photovoltaic cells whose cost in a panel without concentration would be prohibitive.
If a CPV panel is to be mounted to track the sun, then its design is straightforward because the sun will remain aligned with the panel's normal axis. Given the expense and profile that tracking equipment add to solar panel installations, it would be desirable to have a CPV panel that could function in a fixed-position installation and thereby become a viable replacement for the common plate-type solar panels. The creation of such a panel presents a new set of challenges due to the movement of the sun combined with the form-factor constrains of a solar panel. An obvious approach is to fill a shallow enclosure with an array of concentrating elements, using reflective and/or refractive optics, where each element pivots about its own axis or axes to track the sun. Two key performance metrics of such an approach are the fraction of light falling on the panel's face that is captured (its aperture efficiency), and the range of tracking motion of the elements. Obviously a panel that captured 100 percent of the light falling on it and tracked the sun to incidence angles of up to 90 degrees would be the most desirable, but there are numerous issues in existing and proposed designs that impact such performance attributes, and many of these issues arise from constraints imposed by the optical geometry of the CPV elements. The present invention provides a novel reflective optical geometry that solves problems in the design of efficient CPV panels.
CPV systems can be characterized by their concentration ratio, expressed as the number of suns falling on their receivers. Systems with concentration ratios greater than about three generally require the use of moving optical focusing elements that track the movement of the sun across the sky. Such optical tracking systems fall into two main types: systems in which elongate elements with uniform cross-sectional profiles individually tilt about their long axis to track the sun and keep its light focused onto narrow bands, and systems in which elements with radial symmetry individually or as clusters pivot about two axes to track the sun and keep its light focused onto small spots. The size of the disk of the sun, appearing about one-half degree in diameter, limits the theoretical concentration ratio achievable with one-axis and two-axis systems to a few hundred and a few tens of thousands respectively, with practical limits being considerably less due to imperfections in optics and tracking.
The terms one-axis and two-axis are used to denote these two approaches, referring to the number of tilt axes required to keep the sunlight focused. The present invention is applicable to both approaches, and exemplary embodiments falling under both approaches are disclosed herein.
The invention is suitable for systems using photovoltaic, thermal, and hybrid receivers over a variety of scales and a range of concentration ratios. However, because the invention allows the creation of systems having attributes of system efficiency, heat dissipation, and compactness that are particularly suited to the application area of CPV panels, comparisons with prior art made herein focus on attributes relevant to the performance of close-packed arrays of CPV elements.
This review focuses on prior art in that application area of CPV panels. In particular, it examines one- and two-axis tracking systems that employ multiple CPV elements, using primarily reflective optics, each mounted to tilt about its individual axis or axes, and each incorporating an optical focusing means and photovoltaic receiver. An example is described and its drawbacks characterized for each of three such types of such systems: ones in which elements are slats with asymmetric profiles, ones in which the elements are troughs with symmetric profiles, and ones in which the elements are dishes.
U.S. patent application Ser. No. 12/156,189 describes an array of pivotably-mounted slats, each mounting a photovoltaic strip straddling the focal line of a parabolic cylinder mirror on the facing side of the same slat.
Since the focal line of a parabolic mirror is separated from points on the mirror's surface by a distance of at least the parabolic cylinder's focal length, the slat must contain a riser extending above the cylinder's surface to support the strip of photovoltaic material. If the mirror portion of the slat is to be used as a heat sink to wick heat from the photovoltaic strip, then the riser must conduct heat to the mirror as well as provide the structural function of rigidly mounting the strip relative to the mirror, imposing a cost in materials and space requirements.
The same invention also has a limitation in the range of angles through which the slats can rotate, and hence the range of angles of directional incident light projected into a plane perpendicular to the slats' axes through which the system can capture that light and operate, that range being from near 90 degrees clockwise of the normal direction to only between about 10 to 30 degrees counterclockwise of it for most practical variants. This limitation in the system's coverage of directions of incident light constrains the choices for its optimal siting, possibly sacrificing coverage for parts of the diurnal and annual cycles.
U.S. patent application Ser. No. 11/654,256 describes a panel having a series of elongate modules mounted within a frame to pivot about their individual axes. Each module has a strip of photovoltaic cells located along its bottom, symmetrically disposed reflectors forming its sides, and a transparent cover with a central lens forming its top, such that parallel light entering a properly tilted module will either pass through the non-lens portion of the cover and be reflected from the side wall to the photovoltaic strip, or pass through the lens portion of the cover and be refracted to the photovoltaic strip.
Because of the geometric constraints imposed by this optical system, the modules' height-to-width ratio is greater than one, and the bottom of the trough is displaced from the mid-line of the module's aperture-defining transparent cover by a distance of about three times that of either edge of the cover. As a consequence, for the modules to have any appreciable range of motion, they must be spaced such that a portion of directional light perpendicular to the panel falls between modules, decreasing the panel's effective aperture. Increasing the modules' range of motion increases the required spacing interval, further increasing aperture losses. To accommodate a range of motion of 70 degrees to either side of the panel's perpendicular direction entails an aperture loss of up to about 50 percent.
U.S. patent application Ser. No. 11/454,441 describes a panel in which each of an array of CPV elements is pivotably mounted in a base structure and articulated to a moving bracket that forces the elements to move in unison about two axes to track the sun. The body of each CPV element is formed primarily from two halves, each half comprising a paraboloid reflector portion and flat, vertical portions, where the reflector faces of the two halves belong to the same paraboloid, whose focus is situated between the halves' vertical portions.
Details of the mechanical linkages that articulate the elements to the moving bracket and frame are not well specified, nor is the range of angular motion of the elements given the linkage. It appears that a space between each element's said halves enables a significant range of motion along one axis, but at the expense of a substantial loss of aperture. The design uses the vertical portions of the CPV element's halves both to support its photovoltaic cell, and provide surfaces for heat dissipation. Although thin in profile, these features in the “optical volume” of the element further reduce its aperture. Still more loss of aperture results from the fact that light reflected from portions of the reflector near the bases of the vertical portions is blocked from reaching the cell by that portion.
Apart from these issues, the design suffers from a trade-off between the receiver incidence angles and the CPV elements' tracking range that is shared by other designs that mount a receiver in the space above a paraboloid dish. In order for the CPV element to have a wide range of angular motion, the structure supporting the receiver must be relatively short so as not to collide with adjacent elements, resulting in a high average incidence angle of reflected light on the receiver. Because the efficiency of light capture by most photovoltaic cells begins to fall off for incidence angles of more than about 45 degrees, the fact that a significant fraction of the light captured by CPV elements of said type reaches the receiver at incidence angles in the vicinity of 45 degrees represents a potentially significant detriment to the system's efficiency.
Most two-axis CPV panel designs employ refractive instead of reflective optics. Refractive optics used for solar concentration, however, has a number of disadvantageous characteristics, including the spreading of focal spots due to chromatic aberration, the susceptibility of optical plastics to UV degradation and the weight of optical glasses, and, as with paraboloid dishes, high average incidence angles on receivers for designs in which the modules are sufficiently compact to have a wide range of angular motion.